Introduction of a new friction routine into SWAN model that evaluates rougness due to bedform and sediment size changes

The interaction between wave energy and the seabed results in a dissipation of energy that is due to bottom friction. Work is also done on bedforms both solid and mobile (cohesive and noncohesive sediments) such as sand ripples, on suspending and moving sediment, due to percolation (Komen et al. 1994), and due to excessive breaking caused by shoaling (e.g. Babanin et al. 2001). For a relatively flat granular seabed, the magnitude of the roughness that contributes to dissipation is determined by the grain size of the sediment, and dependence of the wave-motion friction on this grain size is one of the subjects of this paper. However, the bedform of a mobile seabed can be altered due to the action of waves and currents. Experimental analysis of flow in the boundary layer found, for example, eddies moving the sediment in an orbital trajectory that results in parallel ridges (or ripples). Their formation and size is determined by the dimension of the eddies (Melnikhova amd Volkov 2000). A bottom-friction routine, based on the Nielsen algorithm (Nielsen 1981), was introduced into the SWAN model (Booij et al. 1999) which makes the bottom friction for waves dependent on presence/absence of ripples if the sea bed is mobile, and on grain size of the sediment. The routine is suitable for spectral models of wave evolution, and in the present study it was tested by means of the SWAN model by hindcasting waves at two finite-depth field sites.